1. Field of the Invention
The present invention relates to a vehicle driving-force control device that applies adaptive control of an automatic transmission to control of an engine having at least two engine modes.
2. Description of the Related Art
In the related art, examples of an automatic transmission include a multistage transmission that switches gear ratios in a stepwise manner and a continuously variable transmission that switches gear ratios in a stepless manner. A known representative type of a multistage transmission is a planetary-pinion type. On the other hand, known types of a continuously variable transmission include a belt or chain based type that uses a metallic belt or a metallic chain as a power shift component and a toroidal type that performs a shift operation by changing a contact radius of a power roller relative to a disk.
Shift control of such an automatic transmission is performed by a transmission control unit (TCU). Specifically, the shift control is performed according to a driving state of a vehicle by referring to a shift map that stores a shift characteristic for each gear ratio, on the basis of parameters such as a vehicle speed and a throttle opening for detecting the driving state.
In this case, a technology of performing so-called driving-condition adaptive control (simply referred to as “adaptive control” hereinafter) is also known, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2010-169111 (referred to as “Patent Document 1” hereinafter). Specifically, in this adaptive control, the TCU performs upshifting or downshifting or performs a shift hold (that is, fixes a gear ratio) according to the driver's intention when the vehicle accelerates or decelerates, or is running along a curved road.
Since the maximum driving force differs between a fuel efficient vehicle and a high performance vehicle such as a turbo vehicle, the maximum acceleration naturally differs therebetween. Therefore, when driving along a slope with continuous curves, for example, an amount of operation (that is, degree of depression) of a brake pedal and an accelerator pedal also differs between a fuel efficient vehicle and a high performance vehicle.
In another similar known technology, a single vehicle has at least two different driving-force characteristics. In this case, when the driver selects one mode from multiple engine modes, engine output characteristics are switched, and a shift characteristic of the automatic transmission is correspondingly changed. For example, Japanese Patent No. 3930529 (referred to as “Patent Document 2” hereinafter) discloses a technology in which a vehicle has three kinds of mode maps with different engine driving-force characteristics. When the driver selects one of the engine modes, the engine is controlled according to the corresponding mode map, and shift control is performed on the basis of a shift pattern that corresponds to the selected engine mode.
In the technology disclosed in Patent Document 2, the engine modes include three modes, which are a normal mode (referred to as “S mode” hereinafter) suitable for normal driving, a save mode (referred to as “I mode” hereinafter) in which a rise in a target torque is reduced relative to the S mode, and a power mode (referred to as “S# mode” hereinafter) in which a rate of change in the target torque relative to a change in the accelerator opening is set to a large value over substantially the entire driving range.
Specifically, as shown in FIG. 6, in the S mode, the degree of opening of a throttle valve (throttle opening SV) changes substantially linearly with respect to the degree of depression of the accelerator pedal (accelerator opening AP). In other words, the characteristic of the S mode is set similar to that of a naturally aspirated DOHC engine. In the I mode, the characteristic thereof is set so as to be suitable for driving at a low-to-medium speed range since the throttle opening SV does not reach full throttle even when the accelerator pedal is fully pressed. In other words, the characteristic of the I mode is set similar to that of an SOHC engine. In contrast, in the S# mode, the characteristic thereof is set such that the rate of change in the throttle opening SV relative to a change in the accelerator opening AP is set to a large value over the entire driving range. Therefore, the characteristic of the S# mode is set similar to that of a turbo engine, which is suitable for sporty driving in which the engine can exhibit its maximum potential.
Accordingly, in the technology disclosed in Patent Document 2, a single vehicle can exhibit the driving performance of three types of vehicles, which are a normal vehicle (NA vehicle), a fuel efficient vehicle, and a high performance vehicle, depending on the selected engine mode.
In the aforementioned adaptive control, if sudden deceleration due to braking is detected during traveling on a road with continuous curves, for example, the automatic transmission is shifted down to prepare for subsequent acceleration. When releasing of the accelerator pedal is subsequently detected, upshifting is performed based on the assumption that the vehicle will make a turn after braking. If the vehicle is running up a slope, downshifting is performed for achieving acceleration performance.
When this is applied to a vehicle having the aforementioned three engine modes, the braking performance of the vehicle is naturally set so as to assure the driving performance of the S# mode that can exhibit the maximum driving-force performance. Therefore, the adaptive control is also set so as to assure the performance of the S# mode.
In this case, since the driving force in the I mode is limited, as mentioned above, the acceleration performance like that of the S# mode cannot be obtained. Therefore, the I mode does not need to have the braking performance like that of the S# mode. In other words, if a braking force similar to that in the S# mode is exhibited in the I mode, the vehicle speed would rapidly decrease.
Therefore, when the vehicle is running in the I mode, the driver drives the vehicle with a reduced braking force so that the speed of the vehicle entering a road with continuous curves is naturally lower than that in the S# mode. As a result, the speed of the vehicle upon making a turn on a curved road in the I mode is entirely lower than that in the S# mode. Consequently, when braking is detected in such a condition and downshifting is performed by the adaptive control, the driver would feel a sense of sudden deceleration beyond the driver's prediction.